Interdigitated multiple pixel arrays of light-emitting devices

ABSTRACT

The present invention discloses a plurality of interdigitated pixels arranged in an array, having a very low series-resistance with improved current spreading and improved heat-sinking. Each pixel is a square with sides of dimension l. The series resistance is minimized by increasing the perimeter of an active region for the pixels. The series resistance is also minimized by shrinking the space between a mesa and n-contact for each pixel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending and commonly-assigned:

U.S. Utility application Ser. No. 14/311,071, filed on Jun. 20, 2014, byArpan Chakraborty, Likun Shen and Umesh K. Mishra, entitled“INTERDIGITATED MULTIPLE PIXEL ARRAYS OF LIGHT-EMITTING DEVICES,” whichapplication is a continuation of co-pending and commonly-assigned:

U.S. Utility application Ser. No. 13/622,979, filed on Sep. 19, 2012, byArpan Chakraborty, Likun Shen and Umesh K. Mishra, entitled“INTERDIGITATED MULTIPLE PIXEL ARRAYS OF LIGHT-EMITTING DEVICES,” nowU.S. Pat. No. 8,796,912, issued Aug. 5, 2014, which application is acontinuation of co-pending and commonly-assigned:

U.S. Utility application Ser. No. 13/045,148, filed on Mar. 10, 2011, byArpan Chakraborty, Likun Shen and Umesh K. Mishra, entitled“INTERDIGITATED MULTIPLE PIXEL ARRAYS OF LIGHT-EMITTING DEVICES,” nowU.S. Pat. No. 8,274,206, issued Sep. 25, 2012, which application is acontinuation of co-pending and commonly-assigned:

U.S. Utility application Ser. No. 12/419,788, filed on Apr. 7, 2009, byArpan Chakraborty, Likun Shen and Umesh K. Mishra, entitled“INTERDIGITATED MULTIPLE PIXEL ARRAYS OF LIGHT-EMITTING DEVICES,” nowU.S. Pat. No. 7,911,126, issued Mar. 22, 2011, which application is acontinuation of co-pending and commonly-assigned:

U.S. Utility application Ser. No. 11/264,794, filed on Nov. 1, 2005, byArpan Chakraborty, Likun Shen and Umesh K. Mishra, entitled“INTERDIGITATED MULTI-PIXEL ARRAYS FOR THE FABRICATION OF LIGHT-EMITTINGDEVICES WITH VERY LOW SERIES-RESISTANCES AND IMPROVED HEAT-SINKING,” nowU.S. Pat. No. 7,518,305, issued Apr. 14, 2009, which application claimsthe benefit under 35 U.S.C. §119(e) of co-pending and commonly-assigned:

U.S. Provisional Application Ser. No. 60/624,026, filed on Nov. 1, 2004,by Arpan Chakraborty, Likun Shen and Umesh K. Mishra, entitled“INTERDIGITATED MULTI-PIXEL ARRAYS FOR THE FABRICATION OF LIGHT-EMITTINGDEVICES WITH VERY LOW SERIES-RESISTANCES,”

all of which applications are incorporated by reference herein.

STATEMENT REGARDING SPONSORED RESEARCH AND DEVELOPMENT

The present invention was made under support from the University ofCalifornia, Santa Barbara, Solid State Lighting and Display Centermember companies, including Stanley Electric Co., Ltd., MitsubishiChemical Corp., Rohm Co., Ltd., Cree, Inc., Matsushita Electric Works,Matsushita Electric Industrial Co., and Seoul Semiconductor Co., Ltd.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to high-power light-emitting devices withvery low series resistance, improved current spreading and improvedheat-sinking.

2. Description of the Related Art

(Note: This application references a number of different publications asindicated throughout the specification by one or more reference numberswithin brackets, e.g., [x]. A list of these different publicationsordered according to these reference numbers can be found below in thesection entitled “References.” Each of these publications isincorporated by reference herein.)

Photonic semiconductor devices fall into three categories: (1) devicesthat convert electrical energy into optical radiation (e.g., lightemitting diodes and laser diodes); (2) devices that detect opticalsignals (e.g., photodetectors); and (3) devices that convert opticalradiation into electrical energy (e.g., photovoltaic devices and solarcells). Although all three kinds of devices have useful applications,the light-emitting device may be the most commonly recognized because ofits application to various consumer products and applications

A light-emitting device is a widely used semiconductor device whose maincharacteristic is that it emits energy in the form of light when acurrent flows through the device. The basic mechanisms by whichlight-emitting devices operate are well understood in this art and areset forth, for example, by Sze, Physics of Semiconductor Devices, 2dEdition (1981) at pages 681-703, which is incorporated by referenceherein.

The wavelength of light (i.e., its color) that can be emitted by a givenmaterial of the light-emitting device is limited by the physicalcharacteristics of that material, specifically its bandgap energy.Bandgap energy is the amount of energy that separates a lower-energyvalence band and a higher energy conduction band in a semiconductor. Thebands are energy states in which carriers (i.e., electrons or holes) canreside in accordance with well-known principles of quantum mechanics.The “band gap” is a range of energies between the conduction and valencebands that are forbidden to the carriers (i.e., the carriers cannotexist in these energy states). Under certain circumstances, whenelectrons and holes cross the bandgap and recombine, they will emitenergy in the form of light. In other words, the frequency ofelectromagnetic radiation (i.e., the color) that can be produced by agiven semiconductor material is a function of that material's bandgapenergy. The full spectrum of wavelength can be covered bysemiconductors.

A light-emitting device typically includes a diode structure (i.e., ap-n junction) with an active region to improve confinement of electronand hole wavefunctions for a better recombination efficiency. The activeregion can comprise a single quantum well and multiple quantum wells, ap-n homojunction, single heterojunction or double heterojunction, with asingle or multiple quantum well structure being the most preferred. Thenature of quantum wells and their advantages are generally wellunderstood in this art as are the nature and advantages ofheterojunctions with respect to homojunctions and vice versa.Accordingly, these will not be described in detail herein other than asnecessary to describe the invention.

High brightness light-emitting devices involve high drive-currentoperation. However, large series-resistance, because of the contacts andthe bulk p-regions and n-regions of the devices, results in heating.This leads to the saturation of the output power at highercontinuous-wave (CW) drive-currents.

Moreover, high brightness light-emitting devices generally require veryhigh output power. However, conventional light emitters employingsemiconductor have a large turn-on voltage as well as a large seriesresistance because of the resistivity of the bulk p-layers and n-layers.This prevents high current operation because the device generates hugeamount of heat with high input electrical power. In addition, the heatgenerated at higher drive currents leads to the roll-off of the outputpower.

What is needed, then, is a design for improved high brightnesslight-emitting devices. Specifically, what is needed is a light-emittingdevice with very low series-resistance and improved heat-sinking,effectively rendering high current operation of the device for highpower light emitting. The present invention satisfies these needs viathe development of a new mask design to minimize the effective seriesresistance, improve current spreading and also allowing better heatsinking.

SUMMARY OF THE INVENTION

The present invention discloses a plurality of interdigitated pixelsarranged in an array, with very low series-resistances, improved currentspreading and heat-sinking, and high brightness output.

In an exemplary embodiment, each pixel is a light emitting diode,wherein the pixels can be of any geometric shape and any size. Inaddition, a light emitting surface comprises either a top or bottom ofthe device.

Each pixel is a square with sides of dimension l. The series resistanceis minimized by increasing the perimeter of an active region for thepixels. The series resistance is also minimized by shrinking the spacebetween a mesa and n-contact for each pixel.

Heat-sinking is improved by separating the pixels by a distance.

Current spreading is improved by shrinking the pixels' size. Inaddition, current spreading is improved by the interdigitated pixels'geometry.

The present invention also discloses a method of fabricating alight-emitting device, comprising fabricating a plurality ofinterdigitated pixels arranged in an array, with very lowseries-resistances, improved current spreading and heat-sinking, andhigh brightness output. In addition, the present invention discloses adevice fabricated according to this method.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 is a schematic cross-section of a light emitting diode structuregrown on insulating substrate, wherein the components of seriesresistance are depicted in the figure.

FIG. 2 is a schematic plan-view of an interdigitated multi-pixelgeometry.

FIG. 3 is a graph illustrating the effects of the number of pixels onthe series resistance of a blue light emitting device for differentdevice dimensions.

FIG. 4A is a schematic of an array of pixels forming a part of an IMPAlight emitting device, wherein the dimensions of the pixel and the busare shown.

FIG. 4B is a schematic plan view of a single square-shaped lightemitting device comprising a die with the dimension of the active areaand the p-pads and n-pads.

FIG. 5 is a graph illustrating the variation of the ratio of theeffective die area (A_(IMPA)/A_(sq)) as a function of the number ofpixel, for different device dimensions (L).

FIG. 6 is a graph that comprises a plot of the series resistance anddie-area product vs. N for a blue light emitting device, to obtain theoptimum number of pixel (N_(opt)).

FIG. 7 is a graph illustrating the ratio of the input power of theconventional single square-shaped light emitting device and the inputpower of an IMPA light emitting device with the optimum number of pixelfor a L=900 μm and L=300 μm.

FIG. 8 is a graph illustrating the normalized current density, i.e.J(x)/J(0), as a function of the distance from the p-contact edge (x) fordifferent p-contact layer thicknesses.

FIG. 9 is a schematic of a flip-chip light emitting device on a sapphiresubstrate.

FIG. 10A is a graph illustrating the temperature distribution inside (A)an IMPA light emitting device and FIG. 10B is a graph illustrating aconventional single square-shaped light emitting device with the sameactive area (L=900 μm) under a drive current of 1 Amp.

FIG. 11 is a graph illustrating the junction temperature of 300×300 μm²IMPA light emitting device as a function of inter-pixel spacing (d),wherein the shaded region indicates the temperature of conventionalsquare-shaped light emitting device under the same drive current.

FIG. 12 is a flow chart that illustrates the steps performed in theprocessing flow to fabricated nitride-based light emitting diodesaccording to the preferred embodiment of the present invention; and

FIG. 13 is a display of an I-V curve from a curve tracer using a mask ona 275 nm deep-UV light emitting device, according to the preferredembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

(I) Analysis for Series Resistance

FIG. 1 illustrates a light-emitting device 10 that comprises a diodestructure (i.e., a p-n junction) fabricated on a substrate or template12 with active layers or region (such as quantum wells (QWs)) 14embedded between p-layers or regions 16 and n-layers or regions 18. Thep-layers 16 include both a p⁻ layer 20 and p⁺ layer 22. The device 10also includes a semi-transparent p-current-spreading layer (i.e.,p-contact) 24, thick p-contact 26 and n-contact 28.

For a square-shaped light-emitting device grown on an insulatingsubstrate, such as shown in FIG. 1, with the n-contact 28 surrounding amesa (comprised of the n-layers 18, active layers 14, p-layers 16,p-current-spreading layer 24 and p-contact 26) from three sides, thetotal series-resistance of the device (R_(s)) is given by the sum of theresistances from the semi-transparent p-current-spreading layer orp-contact 24 resistance (R_(sp)), the p-contact 26 resistance (R_(cp)),the p-cladding layers 16 resistance (R_(bp)), the n-spreading orn-cladding layer 18 resistance (R_(bn)) and the n-contact 28 resistance(R_(cn)).

For a square mesa of sides L, the total series resistance (R_(sq)) isgiven by [10]:R _(sq)=ρ_(sp)/3+ρ_(cp) /L ²+ρ_(p) t _(p) /L ²+ρ_(sn)·(L _(sp)+λ)/3L+ρ_(cn)/3Lwhere, ρ_(sp) is the sheet-resistance of the p-current-spreading layer24, ρ_(cp) is the p-contact 26 resistance, ρ_(p) is the resistivity ofthe p-cladding layers 16, ρ_(sn) is the sheet-resistance of the n-layer18, ρ_(cn) is the n-contact 28 resistance, t_(p) is the thickness of thep-cladding layers 16, λ is the spacing between the mesa and then-contact 28, and d is the width of the n-contact 28. L_(sp) is thespreading length of the n-layer 18 given by [10]:L _(sp)=[(ρ_(p) t _(p)+ρ_(cp))/(|ρ_(sp)−ρ_(sn)|)]^(1/2)

FIG. 2 is a schematic of an interdigitated multi-pixel array (IMPA) 30,which is comprised of a matrix of square-shaped pixels 32 of length l,and comprising N rows and columns. Each square-shaped pixel 32 allowsefficient usage of the chip area compared to a circular shaped pixel asused in a micro-pixel design [11]. The current to the individual pixelsare fed by p-bus 34 and n-bus 36 running parallel to each other. Theeffective active area of the device 30 is given by (N·l)². Theresistance of the entire device 30 is given by the sum of theresistances of N² pixels 32 in parallel:R _(MP)=[ρ_(sp)/3+ρ_(cp) /l ²+ρ_(p) ·t _(p) /l ²+ρ_(sn)·(L _(sp)+λ)/3l+ρ_(cn)/3l]/N ²,ρ_(sp)/3·N ²+ρ_(cp) /L ²+ρ_(p) ·t _(p) /L ²+ρ_(sn)·(L_(sp)+λ)/3N·L+ρ _(cn)/3N·Lwhere L=N·l is the side of a single square-shaped light-emitting devicewith the same effective active area. Since N≧1, therefore R_(MP)≦R_(sq).

FIG. 3 shows the decrease of the series resistance with the increase ofthe number of pixels per row (N) for different device dimension (L).

Table 1 summarizes the values of different parameters used in thesimulations.

TABLE 1 Parameters used in the calculation of R_(s) for a blue LED ρ_(p)0.6 Ω-cm t_(p) 200 nm ρ_(c) 1 × 10⁻³ Ω-cm² ρ_(sp) 10 Ω/□ ρ_(sn) 20 Ω/□ρ_(cn) 5 × 10⁻² Ω/cm d 20 μm

The improvement in the series resistance is mainly achieved by theincrease in the perimeter of the mesa. The drop in resistance rolls offfor large values of N because the series resistance (R_(MP)) is thendominated by the sum of the terms R_(cp) and R_(bp) which areindependent of N. The value of N for which the resistance rolls offincreases with the device dimension (L).

However, the increase in the number of pixels (N) is accompanied by theincrease in the total area of the die.

Consider an IMPA light-emitting device as shown in FIG. 4A with N×Nsquare pixels 32, each pixel 32 with side length of l and a spacing of dbetween the pixels 32. The effective die area of the IMPA light-emittingdevice shown in FIG. 4A is given by:

$\begin{matrix}{A_{IMPA} = {\left\lbrack {{\left( {l + d} \right) \cdot N} + d} \right\rbrack \cdot \left\lbrack {{\left( {{2\; l} + d + d} \right) \cdot {N/2}} + d} \right\rbrack}} \\{= \left\lbrack {{\left( {l + d} \right) \cdot N} + d} \right\rbrack^{2}}\end{matrix}$

However, the effective die area of a single square-shaped light-emittingdevice, as shown in FIG. 4B, and comprised of semi-transparent p-contact24, thick p-contact 26 and n-contact 28, wherein the semi-transparentp-contact 24 has sides of length L and the p-contact 26 and n-contact 28have widths of d, is given by:

$\begin{matrix}{A_{sq} = {\left( {L + {2\; d}} \right) \cdot \left( {L + {2\; d}} \right)}} \\{= \left( {L + {2\; d}} \right)^{2}}\end{matrix}$

Thus, the ratio of the effective die area of an IMPA light-emittingdevice and a single square light-emitting device with same effectiveactive area (L²) is given by:

$\begin{matrix}{{A_{IMPA}A_{sq}} = \left\lbrack {\left\{ {{\left( {l + d} \right) \cdot N} + d} \right\}/\left\{ {L + {2\; d}} \right\}} \right\rbrack^{2}} \\{= \left\lbrack {\left\{ {{l \cdot N} + {\left( {N + 1} \right) \cdot d}} \right\}/\left\{ {L + {2\; d}} \right\}} \right\rbrack^{2}}\end{matrix}$ A_(IMPA)/A_(sq) = [{L + (N + 1) ⋅ d}/{L + 2 d}]²

FIG. 5 shows the variation of the ratio of the effective die area(A_(IMPA)/A_(sq)) as a function of the number of pixel, for differentdevice dimensions (L). The ratio A_(IMPA)/A_(sq) increases with increasein the number of pixel per row (N=L/l). Or, in other words, the ratioincreases by shrinking the size of the individual pixel. We also observethat for a fixed number of pixels (N), the ratio A_(IMPA)/A_(sq)decreases with the increase in the effective active area of the device(L²). Thus, the ratio will be relatively smaller (<3) for larger powerchips (L>500 μm) compared to standard chips (L<500 μm).

Thus, we find from FIG. 3 and FIG. 5 that, for a constant effective area(A=L²), the increase in N results in a decrease in the series resistance(R_(s)) of the light-emitting device but at the expense of increasedeffective die area. To optimize the number of pixel, we determine thevalue of N for which the product of the series resistance (R_(s)) andthe effective die area (A_(IMPA)) is a minimum (see FIG. 6).

The optimum number of pixel (N_(opt)) for different device dimensionsand the corresponding series resistance is given in Table 2.

TABLE 2 Chart for optimized LED design for blue LED: L (μm) N_(opt)R_(S)/R_(MP) 100  2 1.4 200 3-4 2.4 300 6-7 4.9 500 12 10.8 700 16-2018.6 900 21-30 22.9

It can be seen that the N_(opt) increases with the device dimension (L).This signifies that with the increase in the device dimension, themulti-pixel design becomes increasingly beneficial. For large-areadevices, the gain in the resistance by increasing the number of pixelsfor N≦N_(opt), is more significant than the increase in the effectivedie-area. The ratio of the series resistance of a single square-shapedlight-emitting device to the series resistance of the IMPAlight-emitting device for the optimum number of pixel is also given inTable 2. The ratio of the input power of the conventional singlesquare-shaped light-emitting device and the input power of the IMPAlight-emitting device with the optimum number of pixel for a fixeddevice dimension (L) is given by:Λ=P _(sq) /P _(IMPA-opt)=(V ₀ +I·R _(sq))/(V ₀ +I·R _(IMPA))wherein V₀ is the turn-on voltage of the light-emitting device. Theinput power is also equal to the heat dissipated as a result of Jouleheating. The ratio, Λ, is plotted in FIG. 7 for two sets of devicedimension (L) and three sets of turn-on voltage (V₀). It can be observedthat Λ>1 and it increases with the increase in the drive current. Thisimplies that for a fixed L, the multi-pixel design helps in lowering theinput power (as well as heat dissipation due to Joule heating) moreeffectively for higher drive currents compared to smaller drivecurrents. Also, for a fixed drive current, we find that the multi-pixelgeometry for a larger device helps in lowering the input powerconsumption more effectively than a smaller device. For example, theratio Λ=3 signifies that for the same drive current, the IMPAlight-emitting device will consume (or produce) 3 times lower electricalpower (or heat) compared to a standard single square-shapedlight-emitting device of same dimension (L).

(II) Analysis for Current Crowding

Let us analyze lateral current spreading in p-side-up mesa structurelight-emitting devices with a semi-transparent p-contact. The expressionfor the current-density J(x) at a distance x from the p-contact edge isgiven by [10]:J(x)=J(0)·exp(−x/L _(sp))where, J(0) is the current density at the p-type contact edge and L_(sp)is the length where the current density has dropped to 1/e value of thecurrent density at the p-contact edge, i.e. J(L_(sp))/J(0)=1/e. L_(sp)is called the current spreading length of the n-layer and for alight-emitting device with a semi-transparent p-contact, L_(sp) is givenby the expression [10]:L _(sp)=[(ρ_(p) ·t _(p)+ρ_(cp))/(|ρ_(sp)−ρ_(sn)|)]^(1/2)where, ρ_(p) is the resistivity of the p-cladding layer, t_(p) is thethickness of the p-cladding layer, ρ_(cp) is the p-specific contactresistance, ρ_(sp) is the sheet-resistance of the p-current-spreadinglayer, and ρ_(sn) is the sheet-resistance of the n-layer. Using thevalues for the parameters given in Table 1, we plot (see FIG. 8) thenormalized current density, i.e. J(x)/J(0), as a function of thedistance from the p-contact edge (x) for different p-contact layerthicknesses. It can be seen from the figure that the current densitydrops exponentially in the lateral direction and the rate is mainlygoverned by the difference of the sheet resistance of the n-layer andthe p-contact, i.e. (|ρ_(sp)−ρ_(sn)|). As the area of a singlesquare-shaped light-emitting device increases, the current spreadinglength becomes much smaller compared to the device dimension(L_(sp)<<L). This results in poor current spreading in the device andthe light-emitting device is nonuniformly illuminated. Gao et al. [7,8]showed that the use of interdigitated mesa geometry improved currentspreading in light-emitting devices. We propose the use of IMPA geometryto improve lateral current spreading in light-emitting devices. Theindividual pixels allows uniform current spreading by reducing thedimension of the mesa so that L_(sp)>>1. The shaded region in FIG. 8shows the current distribution inside a pixel of l. It can be seen thatby decreasing the pixel size, uniform current spreading can be obtainedwithin the pixel. This should result in uniform illumination of theentire light-emitting device.

(III) Analysis for Self Heating

The temperature distribution inside the interdigitated multi-pixellight-emitting device was studied by means of thermal simulationassuming steady-state heat conduction. The results were compared tothose of a conventional single square-shaped light-emitting device.FEMLAB was used to solve the steady-state heat diffusion equation in2-D. Joule heating in the light-emitting device was assumed to be theheat source and a flip-chip light-emitting device design was considered.The light-emitting device structure assumed in the simulation (shown inFIG. 9) was comprised of a 100 μm thick sapphire substrate, 4 μm of GaNbuffer followed by 1 μm of active region acting as the heat source.Thick metals were used as the solder. Si of thickness 600 μm was used asthe submount and it was assumed that the free boundary of the Sisubmount was at 25° C. The temperature distribution inside an IMPAlight-emitting device and a conventional single square-shapedlight-emitting device with the same active area (L=900 μm) and under thesame drive current (1 A) is shown in FIGS. 10A and 10B. It can be seenthat the temperature inside the IMPA light-emitting device (40° C.) isless than half compared to the temperature of the square-shapedlight-emitting device (88° C.). In FIG. 11, we have plotted the junctiontemperature of 300×300 μm² IMPA light-emitting device with differentspacing (d), spacing d=0 being a conventional single square-shapedlight-emitting device (shown by the shaded region). It was observed thatthere is a significant reduction in the junction temperature onincreasing the spacing from d=0 to d=5 μm. This improvement is mainlydue to the decrease in the series resistance by implementing IMPAgeometry. However, the decrease in the junction temperature for d≧5 μmis due the presence of available area which helps to dissipate the heatgenerated in the individual pixels. For our actual designs, we haveadopted a d=20 μm in order to allow enough space for the n-contact stubsbetween the pixels.

(IV) Fabrication Process

FIG. 12 is a flow chart that illustrates the steps performed in theprocessing flow to fabricate nitride-based light emitting diodesaccording to the preferred embodiment of the present invention.Specifically, the flow chart illustrates a method of fabricating alight-emitting device, comprising the step of fabricating a plurality ofinterdigitated pixels arranged in an array, with very lowseries-resistances and high brightness output, wherein each pixel is alight emitting diode and a light emitting surface comprises either a topor bottom of the device.

Block 38 represents the step of performing a blank SiO₂ deposition (150nm) to protect the p-type aluminum gallium nitride (AlGaN) surface.

Block 40 represents the step of performing a mesa etch.

Block 42 represents the step of performing n-contact formation. In thepreferred embodiment, this step includes the deposition of the followinglayers: 20 nm of titanium (Ti), 240 nm of aluminum (Al), 20 nm of nickel(Ni), and 50 nm of gold (Au), in that order, and then annealing thelayers at 870° C. for 30 seconds.

Block 44 represents the step of performing a p-contact formation. In thepreferred embodiment, this step includes an SiO₂ wet etch; followed bythe deposition of the following layers: 5 nm of Ni and 5 nm of Au, inthat order.

Block 46 represents the step of performing p-pad formation. In thepreferred embodiment, this step includes the deposition of the followinglayers: 20 nm of Ti and 400 nm of Au, in that order.

Block 48 represents the step of performing n-pad formation. In thepreferred embodiment, this step includes the deposition of 20 nm of Tiand 400 nm of Au, in that order.

(V) Key Features

Note that the technical description described above includes several keyfeatures relevant to the fabrication of different kinds of lightemitting devices. These key features include:

1. In the preferred embodiments, the quantum wells can comprise anysemiconductor.

2. It can be applied to both front-side as well as backside verticallight-emitting devices.

3. Use of multi-pixel array of similar active area as a single squareshaped geometry, to increase the perimeter to reduce the seriesresistance.

4. Smaller individual pixels (smaller pixel area) prevent the currentcrowding problem by having uniform current distribution throughout thesmall area.

5. Use lithography techniques with resolution of 1 μm or less to reducethe separation between the mesa and the n-contact for lowering theseries resistance.

6. Have the pixels separated from each other facilitate heat sinking.

7. Scale the inter-pixel separation with the individual pixel size forbetter thermal management.

8. Have interdigitated geometry for better contact scheme.

(VI) Possible Modifications and Variations

The following describes possible modifications and variations of theinvention:

1. The present invention can be used with any semiconductorlight-emitting device.

2. Different pixel sizes and different pixel arrays can be used toimprove heat sinking and maximize the output power.

3. Different pixel geometry like circular, hexagonal, rectangular, etc.,can also be used.

4. Spacing between the pixels can be changed to improve the heatsinking.

5. Can be applied to flip-chipbonded light emitting devices.

(VII) Advantages and Improvements

The key features identified above constitute the most critical and novelelements in the design of the interdigitated multi-pixel array mask forlight emitting device design.

The present invention minimizes the series resistance mainly byshrinking the space between the mesa and the n-contact as well asincreasing the perimeter of the active regions. This is the biggestadvantage of the present mask design over other multi-pixel designs.(See, e.g., Appl. Phys. Lett. 85, 1838 (2004), which is incorporated byreference herein).

Further, the multi-pixel geometry also has lower series resistancecompared to a single stripe-shaped geometry of similar area. This isbecause the resistance of thin p-contact layer for the long stripegeometry is much larger than the metal resistance of the distributedpixels.

In the present invention, an attempt has been made to improve the heatsinking by having a distributed array of pixels. Different pixel sizesand inter-pixel separations have been used. Such a distributed array ofpixels is very important for thermal management of light-emittersoperating at very high CW drive-current levels. The heat generated as aresult of Joule heating needs to get dissipated within the wafer forimproved performance at CW operations.

Finally, the use of the present invention has enabled, for the firsttime, the fabrication of a deep-UV light-emitting device with a seriesresistance as low as 2 Ohm and blue light-emitting device with seriesresistance as low as 0.5 Ohm, for 300 micron device, and that allowsoperating the device at 3.5 Amps direct current. FIG. 13 is a graphillustrating an I-V curve from a curve tracer using a mask on a 275 nmdeep-UV light-emitting device, according to the preferred embodiment ofthe present invention. The device had excellent thermal stability underhigh-current CW operation. Also, it was possible to drive 3.5 Ampsthrough a 200 micron device for more than 1 minute without anydegradation.

REFERENCES

The following references are incorporated by reference herein:

Publications

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U.S. Patents

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Foreign Patent Documents

Publn. Number Date Type 0 817 283 January 1998 EP 2 343 294 May 2000 GBWO99/18617 April 1999 WO

CONCLUSION

This concludes the description of the preferred embodiment of thepresent invention. The foregoing description of one or more embodimentsof the invention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Many modifications andvariations are possible in light of the above teaching. It is intendedthat the scope of the invention be limited not by this detaileddescription, but rather by the claims appended hereto.

What is claimed is:
 1. A light-emitting device, comprising: aninterdigitated multi-pixel array (IMPA) comprised of a matrix of pixels,wherein each of the pixels comprises a light-emitting diode (LED),wherein current is fed to each of the pixels from a p-bus and an n-bus,the p-bus and the n-bus include a first set of interdigitated fingersfeeding the current into rows of the pixels, and the p-bus and the n-businclude a second set of interdigitated fingers feeding the current intocolumns of the pixels, wherein the device has a side dimension of L,such that 100 μm≦L≦900 μm, and wherein each of the rows of pixels iscomprised of a number N of the pixels, such that 1≦N≦40.
 2. The deviceof claim 1, wherein the pixels are square-shaped pixels.
 3. The deviceof claim 2, wherein the matrix of pixels is an N×N matrix of pixels. 4.The device of claim 3, wherein each of the pixels has a side length of land a spacing of d between the pixels, such that an effective die areaA_(IMPA) of the light-emitting device is given by: $\begin{matrix}{A_{IMPA} = {\left\lbrack {{\left( {l + d} \right) \cdot N} + d} \right\rbrack \cdot \left\lbrack {{\left( {{2\; l} + d + d} \right) \cdot {N/2}} + d} \right\rbrack}} \\{= {\left\lbrack {{\left( {l + d} \right) \cdot N} + d} \right\rbrack^{2}.}}\end{matrix}$
 5. The device of claim 4, wherein an optimum number N ofthe pixels increases with the side dimension of L.
 6. The device ofclaim 5, wherein the optimum number N of the pixels for the sidedimension L is given by: L N 100 μm  2 200 μm 3-4 300 μm 6-7 500 μm 12700 μm 16-20 900 μm  21-30.


7. A method of fabricating a light-emitting device, comprising:fabricating an interdigitated multi-pixel array (IMPA) comprised of amatrix of pixels, wherein each of the pixels comprises a light-emittingdiode (LED), wherein current is fed to each of the pixels from a p-busand an n-bus, the p-bus and the n-bus include a first set ofinterdigitated fingers feeding the current into rows of the pixels, andthe p-bus and the n-bus include a second set of interdigitated fingersfeeding the current into columns of the pixels, wherein the device has aside dimension of L, such that 100 μm≦L≦900 μm, and wherein each of therows of pixels is comprised of a number N of the pixels, such that1≦N≦40.
 8. The method of claim 7, wherein the pixels are square-shapedpixels.
 9. The method of claim 8, wherein the matrix of pixels is an N×Nmatrix of pixels.
 10. The method of claim 9, wherein each of the pixelshas a side length of l and a spacing of d between the pixels, such thatan effective die area A_(IMPA) of the light-emitting device is given by:$\begin{matrix}{A_{IMPA} = {\left\lbrack {{\left( {l + d} \right) \cdot N} + d} \right\rbrack \cdot \left\lbrack {{\left( {{2\; l} + d + d} \right) \cdot {N/2}} + d} \right\rbrack}} \\{= {\left\lbrack {{\left( {l + d} \right) \cdot N} + d} \right\rbrack^{2}.}}\end{matrix}$
 11. The method of claim 10, wherein an optimum number N ofthe pixels increases with the side dimension of L.
 12. The method ofclaim 11, wherein the optimum number N of the pixels for the sidedimension L is given by: L N 100 μm  2 200 μm 3-4 300 μm 6-7 500 μm 12700 μm 16-20 900 μm  21-30.